The electrostatic de-dusting methods are based on the attraction exerted on electrically charged dust particles by one or several electrodes brought to a potential of charge opposite to that of the dust particles.
Thus, electrostatic de-dusting installations comprise means for circulating in an enclosure a gaseous fluid loaded with dust particles, a device adapted for electrically charging said dust particles and one or several electrodes adapted for attracting said dust particles.
According to a known technique, the dust particles contained in the gas stream to be purified are electrically charged by producing a corona electric discharge in said gas. To this effect, the gas is made to flow in the interval between a first electrode, made of a conductive pin or of a stretched conductive wire, and a second electrode having a relatively large surface, plane or cylindrical for example, and a potential difference of the order of several tens of kilovolts is applied between said electrodes.
The electrical field in the vicinity of the first electrode, which is very strong, causes the formation in a very small region, called active region, of electronic avalanches which generate a large quantity of ions and electrons. The electrons, which are very mobile, tend to leave rapidly the active region by causing at the edge of said active region the formation of a high concentration of positive or negative ions according to whether the first electrode is positive or negative relative to the second electrode. This ion concentration forms a space charge. The dust particles moving within the space charge region acquire by diffusion or bombardment a charge of same sign as the space charge. The final charge of each dust particle is depending on its size, its time of residence in said region and on the value of the space charge, measured by the product of the quantity of ionized particles per unit volume in the space considered and the charge of said particles.
In the case where the dust particle loaded gas is explosive, such as the atmosphere of a wheat silo where the very fine gluten dust which accumulates in the ambient air provides a very explosive mixture, the creation of a corona discharge is to be prohibited, the smallest spark being potentially the origin of considerable damage.
On the other hand, the efficiency of a corona discharge decreases with increases in the gas temperature in which it is produced. This is due to the thermal agitation of the gaseous fluid molecules. When one of said molecules collides with a negative ion, it causes the detachment of the electron from the latter, which produces an increase of the electronic current of the discharge, with a consequent drop of efficiency of the production of the space charge and the appearance of instabilities in the discharge.
This is why the de-dusting of combustion gases issued from hearths, for example fluidized bed hearths burning coal or reclaimed fuels having a low heat value, by using a corona discharge is practically impossible. For lack of an efficient de-dusting method, it has not been possible hitherto to associate directly such hearths to piston engines or gas turbines without these being rapidly deteriorated by the action of the dust particles.
Electrostatic precipitation de-dusting techniques are also known, where there is no corona discharge but an association of very fine droplets with the dust particles one wishes to eliminate.
Thus, for example, it has been proposed to purify a gaseous stream through a gas-liquid contact by spraying a liquid in a supersonic nozzle fed with compressed air, the resultant atomizate being injected, generally against the current, in a gaseous fluid stream to be purified. The nozzle is brought to a high electric potential relative to the mass of the installation, so that the water droplets coming out from it are charged and stick to the dust particles so as to drive them towards the metallic parts electrically connected to the mass of the installation, thereby providing their separation from the gas. The residual dust particles which are carried with the droplets in the gas stream beyond the nozzles are in turn precipitated on an electrode brought to a convenient electric potential.
A further known technique of said type consists in producing a jet of fine water droplets at the outlet of a nozzle connected to the mass and placed opposite an annular electrode polarized by a high voltage so as to impart to said water droplets a charge of predetermined sign. When the particles to remove from the atmosphere receiving the jet are themselves already electrically charged, the charged water droplets are attracted by said particles and form a mist causing their deposition.
Both of these techniques implement a washing with water of the gas to be purified and therefore do not allow a dry treatment of the gas or any atmosphere where the formation of slurries is to be prohibited. Moreover, they are inefficient as regards atmospheres at temperatures at which the water droplets are vaporized before associating themselves to the particles to be removed.